Use `as_deref` in compiler (but only where it makes sense)

27 files changed, 45 insertions, 63 deletions

diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index 712fb5ac71f..acd7eb69ffc 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -1637,7 +1637,7 @@ fn deny_equality_constraints(
                                     // Remove `Bar` from `Foo::Bar`.
                                     assoc_path.segments.pop();
                                     let len = assoc_path.segments.len() - 1;
-                                    let gen_args = args.as_ref().map(|p| (**p).clone());
+                                    let gen_args = args.as_deref().cloned();
                                     // Build `<Bar = RhsTy>`.
                                     let arg = AngleBracketedArg::Constraint(AssocConstraint {
                                         id: rustc_ast::node_id::DUMMY_NODE_ID,
diff --git a/compiler/rustc_builtin_macros/src/asm.rs b/compiler/rustc_builtin_macros/src/asm.rs
index a1051d990b1..9b32d7027dc 100644
--- a/compiler/rustc_builtin_macros/src/asm.rs
+++ b/compiler/rustc_builtin_macros/src/asm.rs
@@ -560,7 +560,7 @@ fn expand_preparsed_asm(ecx: &mut ExtCtxt<'_>, args: AsmArgs) -> Option<ast::Inl
         let template_snippet = ecx.source_map().span_to_snippet(template_sp).ok();
         template_strs.push((
             template_str,
-            template_snippet.as_ref().map(|s| Symbol::intern(s)),
+            template_snippet.as_deref().map(Symbol::intern),
             template_sp,
         ));
         let template_str = template_str.as_str();
diff --git a/compiler/rustc_builtin_macros/src/format_foreign/printf/tests.rs b/compiler/rustc_builtin_macros/src/format_foreign/printf/tests.rs
index fc7442470ac..df773910dbc 100644
--- a/compiler/rustc_builtin_macros/src/format_foreign/printf/tests.rs
+++ b/compiler/rustc_builtin_macros/src/format_foreign/printf/tests.rs
@@ -100,7 +100,7 @@ fn test_iter() {
     let s = "The %d'th word %% is: `%.*s` %!\n";
     let subs: Vec<_> = iter_subs(s, 0).map(|sub| sub.translate().ok()).collect();
     assert_eq!(
-        subs.iter().map(|ms| ms.as_ref().map(|s| &s[..])).collect::<Vec<_>>(),
+        subs.iter().map(Option::as_deref).collect::<Vec<_>>(),
         vec![Some("{}"), None, Some("{:.*}"), None]
     );
 }
diff --git a/compiler/rustc_builtin_macros/src/format_foreign/shell/tests.rs b/compiler/rustc_builtin_macros/src/format_foreign/shell/tests.rs
index f5f82732f20..93a7afcd6e8 100644
--- a/compiler/rustc_builtin_macros/src/format_foreign/shell/tests.rs
+++ b/compiler/rustc_builtin_macros/src/format_foreign/shell/tests.rs
@@ -39,7 +39,7 @@ fn test_iter() {
     let s = "The $0'th word $$ is: `$WORD` $!\n";
     let subs: Vec<_> = iter_subs(s, 0).map(|sub| sub.translate().ok()).collect();
     assert_eq!(
-        subs.iter().map(|ms| ms.as_ref().map(|s| &s[..])).collect::<Vec<_>>(),
+        subs.iter().map(Option::as_deref).collect::<Vec<_>>(),
         vec![Some("{0}"), None, Some("{WORD}")]
     );
 }
diff --git a/compiler/rustc_codegen_cranelift/build_system/utils.rs b/compiler/rustc_codegen_cranelift/build_system/utils.rs
index 48da64906e2..c627af4e62f 100644
--- a/compiler/rustc_codegen_cranelift/build_system/utils.rs
+++ b/compiler/rustc_codegen_cranelift/build_system/utils.rs
@@ -104,5 +104,5 @@ pub(crate) fn copy_dir_recursively(from: &Path, to: &Path) {
 }
 
 pub(crate) fn is_ci() -> bool {
-    env::var("CI").as_ref().map(|val| &**val) == Ok("true")
+    env::var("CI").as_deref() == Ok("true")
 }
diff --git a/compiler/rustc_codegen_cranelift/src/config.rs b/compiler/rustc_codegen_cranelift/src/config.rs
index e59a0cb0a23..45522fb1a4c 100644
--- a/compiler/rustc_codegen_cranelift/src/config.rs
+++ b/compiler/rustc_codegen_cranelift/src/config.rs
@@ -2,7 +2,7 @@ use std::env;
 use std::str::FromStr;
 
 fn bool_env_var(key: &str) -> bool {
-    env::var(key).as_ref().map(|val| &**val) == Ok("1")
+    env::var(key).as_deref() == Ok("1")
 }
 
 /// The mode to use for compilation.
diff --git a/compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs b/compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs
index 8a8d889a298..86580d05d41 100644
--- a/compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs
+++ b/compiler/rustc_codegen_llvm/src/coverageinfo/mapgen.rs
@@ -174,7 +174,7 @@ impl CoverageMapGenerator {
         counter_regions.sort_unstable_by_key(|(_counter, region)| *region);
         for (counter, region) in counter_regions {
             let CodeRegion { file_name, start_line, start_col, end_line, end_col } = *region;
-            let same_file = current_file_name.as_ref().map_or(false, |p| *p == file_name);
+            let same_file = current_file_name.map_or(false, |p| p == file_name);
             if !same_file {
                 if current_file_name.is_some() {
                     current_file_id += 1;
diff --git a/compiler/rustc_driver/src/lib.rs b/compiler/rustc_driver/src/lib.rs
index e043368fdfe..145d93d52d1 100644
--- a/compiler/rustc_driver/src/lib.rs
+++ b/compiler/rustc_driver/src/lib.rs
@@ -318,7 +318,7 @@ fn run_compiler(
                             compiler.input(),
                             &*expanded_crate,
                             *ppm,
-                            compiler.output_file().as_ref().map(|p| &**p),
+                            compiler.output_file().as_deref(),
                         );
                         Ok(())
                     })?;
@@ -329,7 +329,7 @@ fn run_compiler(
                         compiler.input(),
                         &krate,
                         *ppm,
-                        compiler.output_file().as_ref().map(|p| &**p),
+                        compiler.output_file().as_deref(),
                     );
                 }
                 trace!("finished pretty-printing");
@@ -383,10 +383,7 @@ fn run_compiler(
                             &crate_name,
                             compiler.input(),
                             None,
-                            DumpHandler::new(
-                                compiler.output_dir().as_ref().map(|p| &**p),
-                                &crate_name,
-                            ),
+                            DumpHandler::new(compiler.output_dir().as_deref(), &crate_name),
                         )
                     });
                 }
diff --git a/compiler/rustc_errors/src/emitter.rs b/compiler/rustc_errors/src/emitter.rs
index 55c7997a513..d5b595feca9 100644
--- a/compiler/rustc_errors/src/emitter.rs
+++ b/compiler/rustc_errors/src/emitter.rs
@@ -248,7 +248,7 @@ pub trait Emitter: Translate {
         fluent_args: &FluentArgs<'_>,
     ) -> (MultiSpan, &'a [CodeSuggestion]) {
         let mut primary_span = diag.span.clone();
-        let suggestions = diag.suggestions.as_ref().map_or(&[][..], |suggestions| &suggestions[..]);
+        let suggestions = diag.suggestions.as_deref().unwrap_or(&[]);
         if let Some((sugg, rest)) = suggestions.split_first() {
             let msg = self.translate_message(&sugg.msg, fluent_args);
             if rest.is_empty() &&
diff --git a/compiler/rustc_hir_analysis/src/collect.rs b/compiler/rustc_hir_analysis/src/collect.rs
index 0e7a5ebf5ab..916a39cda16 100644
--- a/compiler/rustc_hir_analysis/src/collect.rs
+++ b/compiler/rustc_hir_analysis/src/collect.rs
@@ -2179,7 +2179,7 @@ fn check_link_ordinal(tcx: TyCtxt<'_>, attr: &ast::Attribute) -> Option<u16> {
         .emit();
     }
     let meta_item_list = attr.meta_item_list();
-    let meta_item_list: Option<&[ast::NestedMetaItem]> = meta_item_list.as_ref().map(Vec::as_ref);
+    let meta_item_list = meta_item_list.as_deref();
     let sole_meta_list = match meta_item_list {
         Some([item]) => item.literal(),
         Some(_) => {
diff --git a/compiler/rustc_hir_typeck/src/callee.rs b/compiler/rustc_hir_typeck/src/callee.rs
index 2b019c8c9b7..df40871f99f 100644
--- a/compiler/rustc_hir_typeck/src/callee.rs
+++ b/compiler/rustc_hir_typeck/src/callee.rs
@@ -30,7 +30,7 @@ use rustc_trait_selection::infer::InferCtxtExt as _;
 use rustc_trait_selection::traits::error_reporting::DefIdOrName;
 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt as _;
 
-use std::iter;
+use std::{iter, slice};
 
 /// Checks that it is legal to call methods of the trait corresponding
 /// to `trait_id` (this only cares about the trait, not the specific
@@ -227,22 +227,21 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         ] {
             let Some(trait_def_id) = opt_trait_def_id else { continue };
 
-            let opt_input_types = opt_arg_exprs.map(|arg_exprs| {
-                [self.tcx.mk_tup(arg_exprs.iter().map(|e| {
+            let opt_input_type = opt_arg_exprs.map(|arg_exprs| {
+                self.tcx.mk_tup(arg_exprs.iter().map(|e| {
                     self.next_ty_var(TypeVariableOrigin {
                         kind: TypeVariableOriginKind::TypeInference,
                         span: e.span,
                     })
-                }))]
+                }))
             });
-            let opt_input_types = opt_input_types.as_ref().map(AsRef::as_ref);
 
             if let Some(ok) = self.lookup_method_in_trait(
                 call_expr.span,
                 method_name,
                 trait_def_id,
                 adjusted_ty,
-                opt_input_types,
+                opt_input_type.as_ref().map(slice::from_ref),
             ) {
                 let method = self.register_infer_ok_obligations(ok);
                 let mut autoref = None;
diff --git a/compiler/rustc_hir_typeck/src/coercion.rs b/compiler/rustc_hir_typeck/src/coercion.rs
index 71949b42118..0f74edcf1c8 100644
--- a/compiler/rustc_hir_typeck/src/coercion.rs
+++ b/compiler/rustc_hir_typeck/src/coercion.rs
@@ -589,7 +589,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             }
             _ => None,
         };
-        let coerce_source = reborrow.as_ref().map_or(source, |&(_, ref r)| r.target);
+        let coerce_source = reborrow.as_ref().map_or(source, |(_, r)| r.target);
 
         // Setup either a subtyping or a LUB relationship between
         // the `CoerceUnsized` target type and the expected type.
diff --git a/compiler/rustc_interface/src/interface.rs b/compiler/rustc_interface/src/interface.rs
index 89aaa0b95e4..99c934862c4 100644
--- a/compiler/rustc_interface/src/interface.rs
+++ b/compiler/rustc_interface/src/interface.rs
@@ -304,7 +304,7 @@ pub fn run_compiler<R: Send>(config: Config, f: impl FnOnce(&Compiler) -> R + Se
                 parse_sess_created(&mut sess.parse_sess);
             }
 
-            let temps_dir = sess.opts.unstable_opts.temps_dir.as_ref().map(|o| PathBuf::from(&o));
+            let temps_dir = sess.opts.unstable_opts.temps_dir.as_deref().map(PathBuf::from);
 
             let compiler = Compiler {
                 sess: Lrc::new(sess),
diff --git a/compiler/rustc_interface/src/queries.rs b/compiler/rustc_interface/src/queries.rs
index 91d180e1eb7..fc0b11183f7 100644
--- a/compiler/rustc_interface/src/queries.rs
+++ b/compiler/rustc_interface/src/queries.rs
@@ -33,11 +33,7 @@ pub struct Query<T> {
 
 impl<T> Query<T> {
     fn compute<F: FnOnce() -> Result<T>>(&self, f: F) -> Result<&Query<T>> {
-        let mut result = self.result.borrow_mut();
-        if result.is_none() {
-            *result = Some(f());
-        }
-        result.as_ref().unwrap().as_ref().map(|_| self).map_err(|err| *err)
+        self.result.borrow_mut().get_or_insert_with(f).as_ref().map(|_| self).map_err(|&err| err)
     }
 
     /// Takes ownership of the query result. Further attempts to take or peek the query
diff --git a/compiler/rustc_interface/src/util.rs b/compiler/rustc_interface/src/util.rs
index 2fe3fb2fa56..4142964a0da 100644
--- a/compiler/rustc_interface/src/util.rs
+++ b/compiler/rustc_interface/src/util.rs
@@ -68,10 +68,7 @@ pub fn create_session(
     let codegen_backend = if let Some(make_codegen_backend) = make_codegen_backend {
         make_codegen_backend(&sopts)
     } else {
-        get_codegen_backend(
-            &sopts.maybe_sysroot,
-            sopts.unstable_opts.codegen_backend.as_ref().map(|name| &name[..]),
-        )
+        get_codegen_backend(&sopts.maybe_sysroot, sopts.unstable_opts.codegen_backend.as_deref())
     };
 
     // target_override is documented to be called before init(), so this is okay
@@ -260,7 +257,7 @@ pub fn rustc_path<'a>() -> Option<&'a Path> {
 
     const BIN_PATH: &str = env!("RUSTC_INSTALL_BINDIR");
 
-    RUSTC_PATH.get_or_init(|| get_rustc_path_inner(BIN_PATH)).as_ref().map(|v| &**v)
+    RUSTC_PATH.get_or_init(|| get_rustc_path_inner(BIN_PATH)).as_deref()
 }
 
 fn get_rustc_path_inner(bin_path: &str) -> Option<PathBuf> {
diff --git a/compiler/rustc_lexer/src/unescape/tests.rs b/compiler/rustc_lexer/src/unescape/tests.rs
index c7ca8fd16ae..1c25b03fdb2 100644
--- a/compiler/rustc_lexer/src/unescape/tests.rs
+++ b/compiler/rustc_lexer/src/unescape/tests.rs
@@ -132,8 +132,7 @@ fn test_unescape_str_good() {
                 }
             }
         });
-        let buf = buf.as_ref().map(|it| it.as_ref());
-        assert_eq!(buf, Ok(expected))
+        assert_eq!(buf.as_deref(), Ok(expected))
     }
 
     check("foo", "foo");
@@ -250,8 +249,7 @@ fn test_unescape_byte_str_good() {
                 }
             }
         });
-        let buf = buf.as_ref().map(|it| it.as_ref());
-        assert_eq!(buf, Ok(expected))
+        assert_eq!(buf.as_deref(), Ok(expected))
     }
 
     check("foo", b"foo");
diff --git a/compiler/rustc_metadata/src/creader.rs b/compiler/rustc_metadata/src/creader.rs
index d4c457975a8..1a2389c7a84 100644
--- a/compiler/rustc_metadata/src/creader.rs
+++ b/compiler/rustc_metadata/src/creader.rs
@@ -162,7 +162,7 @@ impl CStore {
     pub(crate) fn iter_crate_data(&self) -> impl Iterator<Item = (CrateNum, &CrateMetadata)> {
         self.metas
             .iter_enumerated()
-            .filter_map(|(cnum, data)| data.as_ref().map(|data| (cnum, &**data)))
+            .filter_map(|(cnum, data)| data.as_deref().map(|data| (cnum, data)))
     }
 
     fn push_dependencies_in_postorder(&self, deps: &mut Vec<CrateNum>, cnum: CrateNum) {
diff --git a/compiler/rustc_metadata/src/rmeta/encoder.rs b/compiler/rustc_metadata/src/rmeta/encoder.rs
index 6a73e14e9f5..ec0f9ad586d 100644
--- a/compiler/rustc_metadata/src/rmeta/encoder.rs
+++ b/compiler/rustc_metadata/src/rmeta/encoder.rs
@@ -2189,7 +2189,7 @@ impl EncodedMetadata {
 
     #[inline]
     pub fn raw_data(&self) -> &[u8] {
-        self.mmap.as_ref().map(|mmap| mmap.as_ref()).unwrap_or_default()
+        self.mmap.as_deref().unwrap_or_default()
     }
 }
 
diff --git a/compiler/rustc_mir_build/src/thir/cx/expr.rs b/compiler/rustc_mir_build/src/thir/cx/expr.rs
index c4639d3a513..139c276346d 100644
--- a/compiler/rustc_mir_build/src/thir/cx/expr.rs
+++ b/compiler/rustc_mir_build/src/thir/cx/expr.rs
@@ -487,7 +487,7 @@ impl<'tcx> Cx<'tcx> {
                             substs,
                             user_ty,
                             fields: self.field_refs(fields),
-                            base: base.as_ref().map(|base| FruInfo {
+                            base: base.map(|base| FruInfo {
                                 base: self.mirror_expr(base),
                                 field_types: self.typeck_results().fru_field_types()[expr.hir_id]
                                     .iter()
@@ -590,7 +590,7 @@ impl<'tcx> Cx<'tcx> {
                             InlineAsmOperand::Out {
                                 reg,
                                 late,
-                                expr: expr.as_ref().map(|expr| self.mirror_expr(expr)),
+                                expr: expr.map(|expr| self.mirror_expr(expr)),
                             }
                         }
                         hir::InlineAsmOperand::InOut { reg, late, ref expr } => {
@@ -605,7 +605,7 @@ impl<'tcx> Cx<'tcx> {
                             reg,
                             late,
                             in_expr: self.mirror_expr(in_expr),
-                            out_expr: out_expr.as_ref().map(|expr| self.mirror_expr(expr)),
+                            out_expr: out_expr.map(|expr| self.mirror_expr(expr)),
                         },
                         hir::InlineAsmOperand::Const { ref anon_const } => {
                             let anon_const_def_id = tcx.hir().local_def_id(anon_const.hir_id);
@@ -659,13 +659,11 @@ impl<'tcx> Cx<'tcx> {
 
                 ExprKind::Repeat { value: self.mirror_expr(v), count: *count }
             }
-            hir::ExprKind::Ret(ref v) => {
-                ExprKind::Return { value: v.as_ref().map(|v| self.mirror_expr(v)) }
-            }
+            hir::ExprKind::Ret(ref v) => ExprKind::Return { value: v.map(|v| self.mirror_expr(v)) },
             hir::ExprKind::Break(dest, ref value) => match dest.target_id {
                 Ok(target_id) => ExprKind::Break {
                     label: region::Scope { id: target_id.local_id, data: region::ScopeData::Node },
-                    value: value.as_ref().map(|value| self.mirror_expr(value)),
+                    value: value.map(|value| self.mirror_expr(value)),
                 },
                 Err(err) => bug!("invalid loop id for break: {}", err),
             },
diff --git a/compiler/rustc_mir_build/src/thir/pattern/mod.rs b/compiler/rustc_mir_build/src/thir/pattern/mod.rs
index 80b532aec6c..68599755f33 100644
--- a/compiler/rustc_mir_build/src/thir/pattern/mod.rs
+++ b/compiler/rustc_mir_build/src/thir/pattern/mod.rs
@@ -216,7 +216,7 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
                 let lo = lo_expr.map(|e| self.lower_range_expr(e));
                 let hi = hi_expr.map(|e| self.lower_range_expr(e));
 
-                let (lp, hp) = (lo.as_ref().map(|x| &x.0), hi.as_ref().map(|x| &x.0));
+                let (lp, hp) = (lo.as_ref().map(|(x, _)| x), hi.as_ref().map(|(x, _)| x));
                 let mut kind = match self.normalize_range_pattern_ends(ty, lp, hp) {
                     Some((lc, hc)) => self.lower_pattern_range(ty, lc, hc, end, lo_span),
                     None => {
@@ -358,7 +358,7 @@ impl<'a, 'tcx> PatCtxt<'a, 'tcx> {
         &mut self,
         pat: &'tcx Option<&'tcx hir::Pat<'tcx>>,
     ) -> Option<Box<Pat<'tcx>>> {
-        pat.as_ref().map(|p| self.lower_pattern(p))
+        pat.map(|p| self.lower_pattern(p))
     }
 
     fn slice_or_array_pattern(
diff --git a/compiler/rustc_parse/src/parser/diagnostics.rs b/compiler/rustc_parse/src/parser/diagnostics.rs
index 0bbe073fe2a..4b059d4443c 100644
--- a/compiler/rustc_parse/src/parser/diagnostics.rs
+++ b/compiler/rustc_parse/src/parser/diagnostics.rs
@@ -2562,7 +2562,7 @@ impl<'a> Parser<'a> {
             if let [a, b] = segments {
                 let (a_span, b_span) = (a.span(), b.span());
                 let between_span = a_span.shrink_to_hi().to(b_span.shrink_to_lo());
-                if self.span_to_snippet(between_span).as_ref().map(|a| &a[..]) == Ok(":: ") {
+                if self.span_to_snippet(between_span).as_deref() == Ok(":: ") {
                     return Err(DoubleColonInBound {
                         span: path.span.shrink_to_hi(),
                         between: between_span,
diff --git a/compiler/rustc_passes/src/liveness.rs b/compiler/rustc_passes/src/liveness.rs
index c181de48a9a..1afcf546e30 100644
--- a/compiler/rustc_passes/src/liveness.rs
+++ b/compiler/rustc_passes/src/liveness.rs
@@ -923,8 +923,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
                 //    v     v
                 //   (  succ  )
                 //
-                let else_ln =
-                    self.propagate_through_opt_expr(else_opt.as_ref().map(|e| &**e), succ);
+                let else_ln = self.propagate_through_opt_expr(else_opt.as_deref(), succ);
                 let then_ln = self.propagate_through_expr(&then, succ);
                 let ln = self.live_node(expr.hir_id, expr.span);
                 self.init_from_succ(ln, else_ln);
@@ -967,7 +966,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
 
             hir::ExprKind::Ret(ref o_e) => {
                 // Ignore succ and subst exit_ln.
-                self.propagate_through_opt_expr(o_e.as_ref().map(|e| &**e), self.exit_ln)
+                self.propagate_through_opt_expr(o_e.as_deref(), self.exit_ln)
             }
 
             hir::ExprKind::Break(label, ref opt_expr) => {
@@ -982,7 +981,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
                 // look it up in the break loop nodes table
 
                 match target {
-                    Some(b) => self.propagate_through_opt_expr(opt_expr.as_ref().map(|e| &**e), b),
+                    Some(b) => self.propagate_through_opt_expr(opt_expr.as_deref(), b),
                     None => span_bug!(expr.span, "`break` to unknown label"),
                 }
             }
@@ -1027,7 +1026,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             hir::ExprKind::Array(ref exprs) => self.propagate_through_exprs(exprs, succ),
 
             hir::ExprKind::Struct(_, ref fields, ref with_expr) => {
-                let succ = self.propagate_through_opt_expr(with_expr.as_ref().map(|e| &**e), succ);
+                let succ = self.propagate_through_opt_expr(with_expr.as_deref(), succ);
                 fields
                     .iter()
                     .rev()
diff --git a/compiler/rustc_save_analysis/src/dump_visitor.rs b/compiler/rustc_save_analysis/src/dump_visitor.rs
index df5d992f663..4fc42cdea36 100644
--- a/compiler/rustc_save_analysis/src/dump_visitor.rs
+++ b/compiler/rustc_save_analysis/src/dump_visitor.rs
@@ -1029,7 +1029,7 @@ impl<'tcx> DumpVisitor<'tcx> {
                                 trait_item.hir_id(),
                                 trait_item.ident,
                                 Some(bounds),
-                                default_ty.as_ref().map(|ty| &**ty),
+                                default_ty.as_deref(),
                                 &self.save_ctxt,
                             ),
                             attributes: lower_attributes(attrs.to_vec(), &self.save_ctxt),
diff --git a/compiler/rustc_session/src/config.rs b/compiler/rustc_session/src/config.rs
index be084adb7b7..1ce3a613dc7 100644
--- a/compiler/rustc_session/src/config.rs
+++ b/compiler/rustc_session/src/config.rs
@@ -1480,7 +1480,7 @@ pub fn get_cmd_lint_options(
 
 /// Parses the `--color` flag.
 pub fn parse_color(matches: &getopts::Matches) -> ColorConfig {
-    match matches.opt_str("color").as_ref().map(|s| &s[..]) {
+    match matches.opt_str("color").as_deref() {
         Some("auto") => ColorConfig::Auto,
         Some("always") => ColorConfig::Always,
         Some("never") => ColorConfig::Never,
@@ -1589,7 +1589,7 @@ pub fn parse_error_format(
     // is unstable, it will not be present. We have to use `opts_present` not
     // `opt_present` because the latter will panic.
     let error_format = if matches.opts_present(&["error-format".to_owned()]) {
-        match matches.opt_str("error-format").as_ref().map(|s| &s[..]) {
+        match matches.opt_str("error-format").as_deref() {
             None | Some("human") => {
                 ErrorOutputType::HumanReadable(HumanReadableErrorType::Default(color))
             }
diff --git a/compiler/rustc_session/src/session.rs b/compiler/rustc_session/src/session.rs
index 10352198357..934355bb14f 100644
--- a/compiler/rustc_session/src/session.rs
+++ b/compiler/rustc_session/src/session.rs
@@ -1354,7 +1354,7 @@ pub fn build_session(
         let profiler = SelfProfiler::new(
             directory,
             sopts.crate_name.as_deref(),
-            sopts.unstable_opts.self_profile_events.as_ref().map(|xs| &xs[..]),
+            sopts.unstable_opts.self_profile_events.as_deref(),
             &sopts.unstable_opts.self_profile_counter,
         );
         match profiler {
@@ -1388,7 +1388,7 @@ pub fn build_session(
         local_crate_source_file.map(|path| file_path_mapping.map_prefix(path).0);
 
     let optimization_fuel = Lock::new(OptimizationFuel {
-        remaining: sopts.unstable_opts.fuel.as_ref().map_or(0, |i| i.1),
+        remaining: sopts.unstable_opts.fuel.as_ref().map_or(0, |&(_, i)| i),
         out_of_fuel: false,
     });
     let print_fuel = AtomicU64::new(0);
diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index 322c7104be4..7ccfa600ec3 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -217,9 +217,7 @@ impl RealFileName {
     pub fn local_path(&self) -> Option<&Path> {
         match self {
             RealFileName::LocalPath(p) => Some(p),
-            RealFileName::Remapped { local_path: p, virtual_name: _ } => {
-                p.as_ref().map(PathBuf::as_path)
-            }
+            RealFileName::Remapped { local_path, virtual_name: _ } => local_path.as_deref(),
         }
     }
 
diff --git a/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs b/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
index 82f0440b307..9bfe527647d 100644
--- a/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
+++ b/compiler/rustc_trait_selection/src/traits/error_reporting/on_unimplemented.rs
@@ -460,7 +460,7 @@ impl<'tcx> OnUnimplementedDirective {
         info!("evaluate({:?}, trait_ref={:?}, options={:?})", self, trait_ref, options);
 
         let options_map: FxHashMap<Symbol, String> =
-            options.iter().filter_map(|(k, v)| v.as_ref().map(|v| (*k, v.to_owned()))).collect();
+            options.iter().filter_map(|(k, v)| v.clone().map(|v| (*k, v))).collect();
 
         for command in self.subcommands.iter().chain(Some(self)).rev() {
             if let Some(ref condition) = command.condition && !attr::eval_condition(